Interfaces

Interfaces

What it means

An interface defines a contract — a set of members (methods, properties, events) that any implementing class or struct must provide — without dictating how they’re implemented. Unlike class inheritance, a type can implement multiple interfaces, making interfaces C#‘s primary tool for polymorphism across unrelated class hierarchies.

Examples

public interface IShape
{
    double GetArea();
}

public interface IDrawable
{
    void Draw();
}

// A class can implement multiple interfaces
public class Circle : IShape, IDrawable
{
    public double Radius { get; set; }

    public double GetArea() => Math.PI * Radius * Radius;
    public void Draw() => Console.WriteLine("Drawing a circle");
}

IShape shape = new Circle { Radius = 5 };
Console.WriteLine(shape.GetArea());       // works through the interface reference

List<IShape> shapes = new() { new Circle { Radius = 2 }, new Circle { Radius = 3 } };
double totalArea = shapes.Sum(s => s.GetArea());

Common mistake

Forgetting that interface members are implicitly public and can’t have an access modifier in the implementing class’s declaration matching anything but public — trying to implement an interface method as private fails to satisfy the contract.

public interface IShape
{
    double GetArea();
}

public class Circle : IShape
{
    private double GetArea() => Math.PI * 25;   // Error: Circle does not implement IShape.GetArea()
}

// Fix: interface implementations must be public
public class Circle : IShape
{
    public double GetArea() => Math.PI * 25;
}

Quick practice

1. Can a C# class implement more than one interface?

   Answer

   Yes — unlike single class inheritance, a class can implement any number of interfaces.

2. What access modifier must a class’s implementation of an interface member use?

   Answer

   public — interface members are implicitly public contracts, and implementations must be publicly accessible to satisfy them.

3. Why prefer coding against an interface type (like IShape) rather than a concrete class type?

   Answer

   It decouples your code from any specific implementation — you can swap in any class that implements the interface (for testing, extension, or future changes) without modifying the code that depends on it.